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Green Sand Metalcasting Foundry News

A Study of Cores Types for the Casting Process

Posted by Hill and Griffith Company on Aug 25, 2020 5:04:26 PM

Excerpt from the article in the March 2015 issue of International Journal of Advanced Technology in Engineering and Science by Dhairya S. Deore, et. al.

Casting is the process of producing metal/alloy component parts of desired shapes by pouring the molten metal/alloy into a prepared mold (of that shape) and then allowing the metal/alloy to cool and solidify. The solidified piece of metal/alloy is known as casting. To enhance the casting process, we must improve the quality of sand. Sand muller improves this quality. A core is essentially a body of materials which forms
components of the mold. It possesses sufficient strength to be handled as an independent unit.
 
A core is an obstruction, which when positioned in the mold, naturally does not permit the molten metal to fill up the space occupied by the core. In this way a core produces a hollow casting. Cores are required to create the recesses, undercuts and interior cavities that are often a part of castings. Cores are employed as inserts in a mold to form design features that are otherwise extremely difficult to produce by simple molding. The dry silica sand is used as a basic refractory material for preparing core. This sand withstands high temperature of metal poured in the mold.

Warut Sintapano Core Mold

VI. Sand Core

The forming of holes, internal cavities and other internal surfaces of casting depends on cores. Therefore, cores are defined as the portion of a mold that forms the hollow interior of the casting. Casting is produced in the foundry by pouring molten metal into a mold made to shape of
the component required.
 
Castings play a vital part in all branches of engineering. The flexibility of casting production techniques enable practically, all shapes to be produced. Though naturally, production cost is important. In domestic applications, castings are used for stoves, gates, cookers, radiators, bath, piping for main water supply and drainage. However, the product of casting on a large scale is a sophisticated and capital-intensive
business. Cores are also used in shaping external surfaces of cast products when a pattern is so shaped that it forms a core as an integral part of the mold such as a green sand core.
 
Though this is an economical method of forming cavities in casting, it is limited to shallow and short lengths. But binders suitable for foundry cores must not only hold sand together but must also be sufficiently resistant to high temperature, in order for it to collapse and allow sand to be easily removed from the casting leaving its surface smooth. The binder's ability to collapse on cooling is known as breakdown. This property is very important to core holes, which are in accessible to felting.
 
6.1 Core Requirements
6.1.1 There are seven requirements for cores:
a. For green sand cores, there must be adequate strength for handling.
b. Permeability must be very high to allow for gas to escape.
c. As the casting or molding cools, the core must be weak enough to break down as the material shrinks. Moreover, the core must be easy to remove during shakeout.
d. Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.
e. A smooth surface finish.
f. A minimum generation of gases during metal pouring.
 
6.2 Binders
Special binders are introduced into core sands to add strength. The oldest binder was vegetable oil, however now synthetic oil is used in conjunction with cereal or clay. The core is then baked in a convection oven between 200 and 250 °C (392 and 482 °F). The heat causes the binder to cross-link or polymerize. While this process is simple, the dimensional accuracy is low. Another type of binder process is called the hot-box process, which uses a thermoset and catalyst for a binder. The sand with the binder is packed into a core box that is heated to approximately 230°C (446 °F), which is where the name originated. The binder that touches the hot surface of the core box begins to cure within 10 to 30 seconds. Depending on the type of binder it may require further baking to fully cure.
 
In a similar vein, the cold-box process uses a binder that is hardened through the use of special gases. The binder coated sand is packed into a core box and then sealed so that a curing gas can be introduced. These gases are often toxic (i.e. amine gas) or odorous (i.e. SO2), so special handling systems must be used. However, because high temperatures are not required, the core box can be made from metal, wood,
or plastic. An added benefit is that hollow core can be formed if the gas is introduced via holes in the core surface, which cause only the surface of the core to harden; the remaining sand is then just dumped out to be used again. For example, a cold-box sand casting core binder is sodium silicate, which hardens on exposure to carbon dioxide.
 
Special binders are used in air-set sands to produce core at room temperature. These sands do not require a gas catalyst because organic binders and a curing catalyst are mixed together in the sand, which initiates the curing process. The only disadvantage with this is that after the catalyst is mixed in, there is a short time to use the sand. A third way to produce room temperature cores is by shell molding.

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